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source: relations.py
# Serializer relations
> Bad programmers worry about the code.
> Good programmers worry about data structures and their relationships.
>
> — [Linus Torvalds][cite]
Relational fields are used to represent model relationships. They can be applied to `ForeignKey`, `ManyToManyField` and `OneToOneField` relationships, as well as to reverse relationships, and custom relationships such as `GenericForeignKey`.
---
**Note:** The relational fields are declared in `relations.py`, but by convention you should import them from the `serializers` module, using `from rest_framework import serializers` and refer to fields as `serializers.<FieldName>`.
---
#### Inspecting automatically generated relationships.
When using the `ModelSerializer` class, serializer fields and relationships will be automatically generated for you. Inspecting these automatically generated fields can be a useful tool for determining how to customize the relationship style.
To do so, open the Django shell, using `python manage.py shell`, then import the serializer class, instantiate it, and print the object representation…
>>> from myapp.serializers import AccountSerializer
>>> serializer = AccountSerializer()
>>> print repr(serializer) # Or `print(repr(serializer))` in Python 3.x.
AccountSerializer():
id = IntegerField(label='ID', read_only=True)
name = CharField(allow_blank=True, max_length=100, required=False)
owner = PrimaryKeyRelatedField(queryset=User.objects.all())
# API Reference
In order to explain the various types of relational fields, we'll use a couple of simple models for our examples. Our models will be for music albums, and the tracks listed on each album.
class Album(models.Model):
album_name = models.CharField(max_length=100)
artist = models.CharField(max_length=100)
class Track(models.Model):
album = models.ForeignKey(Album, related_name='tracks')
order = models.IntegerField()
title = models.CharField(max_length=100)
duration = models.IntegerField()
class Meta:
unique_together = ('album', 'order')
ordering = ['order']
def __unicode__(self):
return '%d: %s' % (self.order, self.title)
## StringRelatedField
`StringRelatedField` may be used to represent the target of the relationship using its `__unicode__` method.
For example, the following serializer.
class AlbumSerializer(serializers.ModelSerializer):
tracks = serializers.StringRelatedField(many=True)
class Meta:
model = Album
fields = ('album_name', 'artist', 'tracks')
Would serialize to the following representation.
{
'album_name': 'Things We Lost In The Fire',
'artist': 'Low',
'tracks': [
'1: Sunflower',
'2: Whitetail',
'3: Dinosaur Act',
...
]
}
This field is read only.
**Arguments**:
* `many` - If applied to a to-many relationship, you should set this argument to `True`.
## PrimaryKeyRelatedField
`PrimaryKeyRelatedField` may be used to represent the target of the relationship using its primary key.
For example, the following serializer:
class AlbumSerializer(serializers.ModelSerializer):
tracks = serializers.PrimaryKeyRelatedField(many=True, read_only=True)
class Meta:
model = Album
fields = ('album_name', 'artist', 'tracks')
Would serialize to a representation like this:
{
'album_name': 'The Roots',
'artist': 'Undun',
'tracks': [
89,
90,
91,
...
]
}
By default this field is read-write, although you can change this behavior using the `read_only` flag.
**Arguments**:
* `queryset` - The queryset used for model instance lookups when validating the field input. Relationships must either set a queryset explicitly, or set `read_only=True`.
* `many` - If applied to a to-many relationship, you should set this argument to `True`.
* `allow_null` - If set to `True`, the field will accept values of `None` or the empty string for nullable relationships. Defaults to `False`.
## HyperlinkedRelatedField
`HyperlinkedRelatedField` may be used to represent the target of the relationship using a hyperlink.
For example, the following serializer:
class AlbumSerializer(serializers.ModelSerializer):
tracks = serializers.HyperlinkedRelatedField(
many=True,
read_only=True,
view_name='track-detail'
)
class Meta:
model = Album
fields = ('album_name', 'artist', 'tracks')
Would serialize to a representation like this:
{
'album_name': 'Graceland',
'artist': 'Paul Simon',
'tracks': [
'http://www.example.com/api/tracks/45/',
'http://www.example.com/api/tracks/46/',
'http://www.example.com/api/tracks/47/',
...
]
}
By default this field is read-write, although you can change this behavior using the `read_only` flag.
**Arguments**:
* `view_name` - The view name that should be used as the target of the relationship. If you're using [the standard router classes][routers] this will be a string with the format `<modelname>-detail`. **required**.
* `queryset` - The queryset used for model instance lookups when validating the field input. Relationships must either set a queryset explicitly, or set `read_only=True`.
* `many` - If applied to a to-many relationship, you should set this argument to `True`.
* `allow_null` - If set to `True`, the field will accept values of `None` or the empty string for nullable relationships. Defaults to `False`.
* `lookup_field` - The field on the target that should be used for the lookup. Should correspond to a URL keyword argument on the referenced view. Default is `'pk'`.
* `lookup_url_kwarg` - The name of the keyword argument defined in the URL conf that corresponds to the lookup field. Defaults to using the same value as `lookup_field`.
* `format` - If using format suffixes, hyperlinked fields will use the same format suffix for the target unless overridden by using the `format` argument.
## SlugRelatedField
`SlugRelatedField` may be used to represent the target of the relationship using a field on the target.
For example, the following serializer:
class AlbumSerializer(serializers.ModelSerializer):
tracks = serializers.SlugRelatedField(
many=True,
read_only=True,
slug_field='title'
)
class Meta:
model = Album
fields = ('album_name', 'artist', 'tracks')
Would serialize to a representation like this:
{
'album_name': 'Dear John',
'artist': 'Loney Dear',
'tracks': [
'Airport Surroundings',
'Everything Turns to You',
'I Was Only Going Out',
...
]
}
By default this field is read-write, although you can change this behavior using the `read_only` flag.
When using `SlugRelatedField` as a read-write field, you will normally want to ensure that the slug field corresponds to a model field with `unique=True`.
**Arguments**:
* `slug_field` - The field on the target that should be used to represent it. This should be a field that uniquely identifies any given instance. For example, `username`. **required**
* `queryset` - The queryset used for model instance lookups when validating the field input. Relationships must either set a queryset explicitly, or set `read_only=True`.
* `many` - If applied to a to-many relationship, you should set this argument to `True`.
* `allow_null` - If set to `True`, the field will accept values of `None` or the empty string for nullable relationships. Defaults to `False`.
## HyperlinkedIdentityField
This field can be applied as an identity relationship, such as the `'url'` field on a HyperlinkedModelSerializer. It can also be used for an attribute on the object. For example, the following serializer:
class AlbumSerializer(serializers.HyperlinkedModelSerializer):
track_listing = serializers.HyperlinkedIdentityField(view_name='track-list')
class Meta:
model = Album
fields = ('album_name', 'artist', 'track_listing')
Would serialize to a representation like this:
{
'album_name': 'The Eraser',
'artist': 'Thom Yorke',
'track_listing': 'http://www.example.com/api/track_list/12/',
}
This field is always read-only.
**Arguments**:
* `view_name` - The view name that should be used as the target of the relationship. If you're using [the standard router classes][routers] this will be a string with the format `<model_name>-detail`. **required**.
* `lookup_field` - The field on the target that should be used for the lookup. Should correspond to a URL keyword argument on the referenced view. Default is `'pk'`.
* `lookup_url_kwarg` - The name of the keyword argument defined in the URL conf that corresponds to the lookup field. Defaults to using the same value as `lookup_field`.
* `format` - If using format suffixes, hyperlinked fields will use the same format suffix for the target unless overridden by using the `format` argument.
---
# Nested relationships
Nested relationships can be expressed by using serializers as fields.
If the field is used to represent a to-many relationship, you should add the `many=True` flag to the serializer field.
## Example
For example, the following serializer:
class TrackSerializer(serializers.ModelSerializer):
class Meta:
model = Track
fields = ('order', 'title')
class AlbumSerializer(serializers.ModelSerializer):
tracks = TrackSerializer(many=True, read_only=True)
class Meta:
model = Album
fields = ('album_name', 'artist', 'tracks')
Would serialize to a nested representation like this:
{
'album_name': 'The Grey Album',
'artist': 'Danger Mouse',
'tracks': [
{'order': 1, 'title': 'Public Service Announcement'},
{'order': 2, 'title': 'What More Can I Say'},
{'order': 3, 'title': 'Encore'},
...
],
}
# Custom relational fields
To implement a custom relational field, you should override `RelatedField`, and implement the `.to_representation(self, value)` method. This method takes the target of the field as the `value` argument, and should return the representation that should be used to serialize the target. The `value` argument will typically be a model instance.
If you want to implement a read-write relational field, you must also implement the `.to_internal_value(self, data)` method.
## Example
For, example, we could define a relational field, to serialize a track to a custom string representation, using its ordering, title, and duration.
import time
class TrackListingField(serializers.RelatedField):
def to_representation(self, value):
duration = time.strftime('%M:%S', time.gmtime(value.duration))
return 'Track %d: %s (%s)' % (value.order, value.name, duration)
class AlbumSerializer(serializers.ModelSerializer):
tracks = TrackListingField(many=True)
class Meta:
model = Album
fields = ('album_name', 'artist', 'tracks')
This custom field would then serialize to the following representation.
{
'album_name': 'Sometimes I Wish We Were an Eagle',
'artist': 'Bill Callahan',
'tracks': [
'Track 1: Jim Cain (04:39)',
'Track 2: Eid Ma Clack Shaw (04:19)',
'Track 3: The Wind and the Dove (04:34)',
...
]
}
---
# Further notes
## The `queryset` argument
The `queryset` argument is only ever required for *writable* relationship field, in which case it is used for performing the model instance lookup, that maps from the primitive user input, into a model instance.
In version 2.x a serializer class could *sometimes* automatically determine the `queryset` argument *if* a `ModelSerializer` class was being used.
This behavior is now replaced with *always* using an explicit `queryset` argument for writable relational fields.
Doing so reduces the amount of hidden 'magic' that `ModelSerializer` provides, makes the behavior of the field more clear, and ensures that it is trivial to move between using the `ModelSerializer` shortcut, or using fully explicit `Serializer` classes.
## Reverse relations
Note that reverse relationships are not automatically included by the `ModelSerializer` and `HyperlinkedModelSerializer` classes. To include a reverse relationship, you must explicitly add it to the fields list. For example:
class AlbumSerializer(serializers.ModelSerializer):
class Meta:
fields = ('tracks', ...)
You'll normally want to ensure that you've set an appropriate `related_name` argument on the relationship, that you can use as the field name. For example:
class Track(models.Model):
album = models.ForeignKey(Album, related_name='tracks')
...
If you have not set a related name for the reverse relationship, you'll need to use the automatically generated related name in the `fields` argument. For example:
class AlbumSerializer(serializers.ModelSerializer):
class Meta:
fields = ('track_set', ...)
See the Django documentation on [reverse relationships][reverse-relationships] for more details.
## Generic relationships
If you want to serialize a generic foreign key, you need to define a custom field, to determine explicitly how you want serialize the targets of the relationship.
For example, given the following model for a tag, which has a generic relationship with other arbitrary models:
class TaggedItem(models.Model):
"""
Tags arbitrary model instances using a generic relation.
See: https://docs.djangoproject.com/en/dev/ref/contrib/contenttypes/
"""
tag_name = models.SlugField()
content_type = models.ForeignKey(ContentType)
object_id = models.PositiveIntegerField()
tagged_object = GenericForeignKey('content_type', 'object_id')
def __unicode__(self):
return self.tag
And the following two models, which may be have associated tags:
class Bookmark(models.Model):
"""
A bookmark consists of a URL, and 0 or more descriptive tags.
"""
url = models.URLField()
tags = GenericRelation(TaggedItem)
class Note(models.Model):
"""
A note consists of some text, and 0 or more descriptive tags.
"""
text = models.CharField(max_length=1000)
tags = GenericRelation(TaggedItem)
We could define a custom field that could be used to serialize tagged instances, using the type of each instance to determine how it should be serialized.
class TaggedObjectRelatedField(serializers.RelatedField):
"""
A custom field to use for the `tagged_object` generic relationship.
"""
def to_representation(self, value):
"""
Serialize tagged objects to a simple textual representation.
"""
if isinstance(value, Bookmark):
return 'Bookmark: ' + value.url
elif isinstance(value, Note):
return 'Note: ' + value.text
raise Exception('Unexpected type of tagged object')
If you need the target of the relationship to have a nested representation, you can use the required serializers inside the `.to_representation()` method:
def to_representation(self, value):
"""
Serialize bookmark instances using a bookmark serializer,
and note instances using a note serializer.
"""
if isinstance(value, Bookmark):
serializer = BookmarkSerializer(value)
elif isinstance(value, Note):
serializer = NoteSerializer(value)
else:
raise Exception('Unexpected type of tagged object')
return serializer.data
Note that reverse generic keys, expressed using the `GenericRelation` field, can be serialized using the regular relational field types, since the type of the target in the relationship is always known.
For more information see [the Django documentation on generic relations][generic-relations].
## ManyToManyFields with a Through Model
By default, relational fields that target a ``ManyToManyField`` with a
``through`` model specified are set to read-only.
If you explicitly specify a relational field pointing to a
``ManyToManyField`` with a through model, be sure to set ``read_only``
to ``True``.
## Advanced Hyperlinked fields
If you have very specific requirements for the style of your hyperlinked relationships you can override `HyperlinkedRelatedField`.
There are two methods you'll need to override.
#### get_url(self, obj, view_name, request, format)
This method should return the URL that corresponds to the given object.
May raise a `NoReverseMatch` if the `view_name` and `lookup_field`
attributes are not configured to correctly match the URL conf.
#### get_object(self, queryset, view_name, view_args, view_kwargs)
This method should the object that corresponds to the matched URL conf arguments.
May raise an `ObjectDoesNotExist` exception.
### Example
For example, if all your object URLs used both a account and a slug in the the URL to reference the object, you might create a custom field like this:
class CustomHyperlinkedField(serializers.HyperlinkedRelatedField):
def get_url(self, obj, view_name, request, format):
kwargs = {'account': obj.account, 'slug': obj.slug}
return reverse(view_name, kwargs=kwargs, request=request, format=format)
def get_object(self, queryset, view_name, view_args, view_kwargs):
account = view_kwargs['account']
slug = view_kwargs['slug']
return queryset.get(account=account, slug=slug)
---
# Third Party Packages
The following third party packages are also available.
## DRF Nested Routers
The [drf-nested-routers package][drf-nested-routers] provides routers and relationship fields for working with nested resources.
[cite]: http://lwn.net/Articles/193245/
[reverse-relationships]: https://docs.djangoproject.com/en/dev/topics/db/queries/#following-relationships-backward
[routers]: http://www.django-rest-framework.org/api-guide/routers#defaultrouter
[generic-relations]: https://docs.djangoproject.com/en/dev/ref/contrib/contenttypes/#id1
[2.2-announcement]: ../topics/2.2-announcement.md
[drf-nested-routers]: https://github.com/alanjds/drf-nested-routers
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